A common requirement of amplifiers is high efficiency, low distortion amplification of essentially sine wave signals. In communication and instrumentation applications, the source and load are usually single ended. However, the low distortion requirement is usually best satisfied by a balanced, push-pull circuit configuration. In such circuit configurations, the even harmonic distortion components cancel, leaving the third harmonic component as the dominant distortion source. For amplifiers employing bipolar transistors, this third harmonic component is relatively small.
The usual way to employ push-pull amplifiers with single ended signals is to use a pair of push-pull emitter coupled or grounded emitter transistors for the amplifier, with the input power coupled to the bases through a single ended-to-balanced transformer, and the output power coupled from collectors to load through a balanced to single ended transformer. Examples of this prior art methodology are found in Application Notes 593, 779, 1028 and 1024, Motorola RF Device Data Manual, Volume 2, April, 1988. This arrangement provides the requisite power efficiency and low distortion for frequencies above the low frequency cutoff of the input and output transformers. However, in cases where it is necessary to maintain both constant gain and matched terminations down to DC, conventional transformer coupled circuits cannot be used.
In accordance with the present invention, a push-pull amplifier circuit is provided with a new coupling circuit that connects the push-pull outputs to a single ended load. This circuit delivers the output power of both amplifying devices to the load at high frequencies, provides constant gain and output power down to DC, and, if desired, provides constant bidirectional output impedance matching from RF to DC.
The foregoing and additional features and advantages of the present invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.